The past year has been used in large part to develop new approaches to identify the neural circuitry of pain and analgesia in the dorsal horn. Progress has been made in two areas. First, we have developed two approaches to identify the location and density of axons which contain coexistent neurotransmitters. One approach utilizes masking of immunofluorescence by an immunoperoxidase label. A second approach uses two different immunofluorescent markers. Serotonin, enkephalin and substance P innervation of the ventral horn of the spinal cord was used as a test system. In the ventral horn almost all substance P axons were found to contain serotonin while most of the serotonin axons contained neither substance P nor enkephalin. Analysis of dorsal horn axons with coexistent neurotransmitters has begun. A second area of development is the production of monoclonal antibodies to a subpopulation of dorsal horn neuronal elements. Monoclonal antibodies will allow subpopulations of neurons to be isolated for further analysis. These studies have recently been initiated. A cell suspension of nervous tissue, enriched with thalamic projection neurons, has been harvested and used for immunization of mice. An antibody to thalamic projection neurons will provide further information on the organization of this important system in pain transmission. The development of the thalamic projection system in the rat has been examined as an adjunct to our experiments with monoclonal antibodies. In neonates, few thalamic projection neurons could be labeled prior to postnatal day 10. In the trigeminal system at postnatal day 10, projection neurons appear to be concentrated at the rostral pole of nucleus caudalis. These observations suggest that the trigeminothalamic and spinothalamic systems are late to mature. Noxious information in neonates may thus be carried to higher centers of the neuraxis by a separate neuronal system.